There is a sheet postprocessing apparatus for carrying out a staple binding processing or a processing of mounting a binder or the like for sheets subjected to a processing of copying, forming punch holes or the like. JP-A-2003-160273 discloses a sheet postprocessing apparatus for carrying out a processing of binding staples by accurately aligning punch holes of a bundle of sheets attached with the punch holes. According to the sheet postprocessing apparatus of JP-A-2003-160273, the bundle of sheets attached with the punch holes are positioned to align by a reference of the punch hole by inserting a pin into the hole bored at the sheets and the staple binding processing is carried out in a state of inserting the pin thereto.
When the sheets attached with the punch holes are aligned by the reference of the punch hole by inserting the pin driven by a motor, a solenoid or the like through the punch hole of the sheets attached with the punch holes, when a positional shift of the punch hole of the sheets is large, there is a case in which the pin cannot penetrate through the punch hole, a pin drive mechanism is stopped in the midst of a stroke to bring about an error. Particularly, when a number of the sheets is large, or a thick sheet is loaded as a cover, a load of sliding the pin and the punch hole relative to each other is large and therefore, there is a high possibility of bringing about the above-described error.
When the motor, the solenoid or the like is made to be strong such that the pin drive mechanism is not stopped in the midst of the stroke, a problem of large-sized formation of the apparatus or an increase in cost is posed, or there can be brought about a case in which the pin penetrates through the punch hole in a state of shifting the position of the punch hole to deform the punch hole or bring about a crack.